JPH06125169A - Pre-soldering method - Google Patents

Abstract

PURPOSE:To form solder bumps of short pitch using a solder paste without selective coating by coating the whole face of a circuit board with a solder paste and by reflowing it in a vapor bath at a temperature below a boiling point of an organic flux. CONSTITUTION:The whole face of a circuit board 1 is screen-printed with a solder paste 3. Numeral 3a represents solder particles with the gaps filled with a flux vehicle. When this is reflowed, solder particles on bumps melt down to adhere on pad surfaces and turn into large liquid lumps 4a, while solder particles of the paste 3 which coat pad gaps melt down, but hardly joint each other and never enlarge. When a solder paste remaining on the circuit board is removed using a flux cleaner after temperature fall, a solder bump 4 can be formed on a pad without solder bridges.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soldering process using a solder paste, and more particularly to a pre-soldering method of forming solder bumps on an IC chip, a circuit board or the like.

The solder paste is made by mixing fine particles of a soft wax material such as Pb-Sn solder into a flux vehicle to form a paste, and applying an appropriate amount of the solder paste to a place to be soldered. It is brazed by heating. It has a feature that soldering processing can be performed at a plurality of points at a time, and it is often used for forming bumps described below. The process of heating the solder paste to melt the wax material is called reflow.

When mounting an IC chip face down on a circuit board or chip carrier, solder is built up in advance on both bonding pads (bumps are formed) and the IC chips and the like are placed so that the bumps come into contact with each other. When the substrate is heated by heating, the bumps are fused and the two are connected. The bump can be formed by immersing a substrate in which only the pad surface is exposed in a solder bath, but it can also be formed by attaching a solder paste to the pad and reflowing. A process in which solder is preliminarily placed in a place to be soldered, including the case of forming bumps, is called preliminary soldering.

Further, the reflow may be carried out by placing the substrate in a gas atmosphere saturated with vapor such as perfluorocarbon. If you bring the board into this type of saturated steam, the solder will melt due to the latent heat of solidification of the steam,
The temperature of the substrate does not rise above the ambient temperature, and excessive temperature rise can be avoided. Further, in comparison with the process of dipping in a solder bath, such a reflow process can be called vapor bath dipping, and is sometimes called vapor phase soldering.

[0005]

2. Description of the Related Art A solder paste conventionally used for bump formation is a mixture of fine powder of Pb-Sn eutectic solder having a particle size of several tens of μm or more in a flux vehicle made of rosin or an organic solvent. This is selectively applied to a predetermined position by screen printing and heat-treated to form bumps. In order to avoid flow deformation of the selectively applied solder paste, a viscosity modifier such as hardened castor oil is also added to the flux vehicle, and an activator that removes the oxide film on the metal surface to be processed and the solder grain surface is also added. It is usually done.

[0006]

In the above solder paste, the materials of the flux vehicle and the solder alloy are selected from the viewpoint of improving the adhesion to the metal surface.
The melt resulting from a lump of solder paste is usually a series of lumps of liquid. Therefore, if the solder paste is present outside the pad surface or if the amount of the solder paste is too large, it is easy to cause inconveniences such as formation of a solder bridge during reflow.

Therefore, when the bumps are formed by using the solder paste, it is indispensable to selectively attach the necessary amount of the solder paste only to the pad surface. For that reason, selective application such as screen printing is required. Processing is essential and hinders improvement in throughput.

Furthermore, since the ICs mounted on the circuit board have become highly integrated in recent years and the output terminals have a short pitch, the terminals on the side of the circuit board that receives the ICs have a short pitch, and selective paste application for pre-soldering is required. It is necessary to perform with high accuracy. In other words, short-circuiting defects between adjacent terminals are likely to occur due to insufficient printing accuracy in selective paste application and flow deformation of the paste.

In order to deal with such a situation, the present invention provides a preliminary soldering method capable of forming solder bumps having a short pitch by using a solder paste without performing selective coating.

[0010]

In order to achieve this object, the soldering method of the present invention comprises (a) a composition of, for example, In-40% Pb.
A non-eutectic alloy, the grain size is 1/5 or less of the minimum distance between the pre-solder surfaces and the solder powder and (b) a resin that improves the wetting of the metal surface and the solder melt to be pre-soldered (for example, Rosin) and an organic solvent that dissolves the resin (for example, polyethylene glycol monobutyl ether) is prepared, and the solder paste is applied to the entire surface of the circuit board,
It is characterized in that the reflow is performed in a steam bath at a temperature not higher than the boiling point of the organic solvent.

[0011]

According to the pre-soldering method of the present invention, the solder paste is applied to the entire surface of the substrate and reflowed, so that the solder is applied only to the pad surface to form bumps. According to the knowledge newly obtained by the present inventor, such an effect is produced by satisfying the following three conditions. (1) The wettability of the solder melt should not be too good. (2) The grain system of the solder powder is sufficiently small compared to the distance between the metal surfaces to be pre-soldered. (3) A sufficient amount of the organic solvent in the flux vehicle remains through the reflow process without being rapidly lost by heating.

When these conditions are satisfied, the reaction of the solder particles coalescing and growing into a large-sized melt during the reflow process is extremely slowed down, whereas on the pad metal surface,
Since the area is sufficiently large, it is presumed that the speed at which the solder particles adhere and the melt spreads on the metal surface does not decrease so much.

The fine solder particles and the presence of a large amount of flux liquid during reflow make it difficult for the solder particles to coalesce. In the present invention, the reflow temperature is set to the boiling point of the solvent or lower to secure a sufficient amount of flux during the treatment. Further, the non-eutectic solder is inferior in wettability as compared with the Sn-37Pb solder (eutectic solder), which is considered to be effective in suppressing coalescence of the solder particles.

When the treatment of the present invention is carried out, even if the coalescence of the solder particles progresses to some extent, if the solder bridge is not formed, it is removed by the paste removing treatment after the reflow. Therefore, if the pad spacing is large, bridging is unlikely to occur even if the solder particles are large, whereas if the spacing is small, it is necessary to make the solder particles fine. According to the knowledge obtained by the present inventor, in order to avoid the occurrence of bridges, the solder grain size should not exceed 1/5 of the interval between bumps, and this value is preferably 1/6 or less. .

[0015]

EXAMPLES The solder paste used in the examples has the constitution shown in Table 1.

[0016]

[Table 1] As the solder powder, the one from which the oxide film on the surface is removed is used, and as is clear from the above table, the oxide film removing activator contained in the ordinary solder paste is not included in the solder paste of the present invention. The boiling point of diethylene glycol monobutyl ether in the above table is 230 ° C.

The process of the embodiment using this solder paste is schematically shown in FIG. 1, and will be described below with reference to the same drawing. First, as shown in FIG. 3A, the solder paste 3 is screen-printed to a thickness of 150 μm on the entire surface of the circuit board 1 in which the pads 2 are arranged at a pitch of 300 μm. 3a is a solder grain, and the space between the grains is filled with a flux vehicle.

When this is reflowed in 215 ° C. saturated steam of Fluorinert (trade name, manufactured by Sumitomo 3M Co., Ltd.), which is a kind of perfluorocarbon, the solder particles on the pad melt and the pad surface as shown in FIG. While it becomes a large liquid mass 4a, the solder particles of the solder paste 3 applied between the pads melt, but the coalescence hardly progresses and the particles do not become large.

When the solder paste remaining on the circuit board is removed using a flux cleaning agent after the temperature has dropped, solder bumps 4 are formed on the pads without the occurrence of solder bridges, as shown in FIG. Has been done.

[0020]

As described above, according to the pre-soldering method of the present invention, it is possible to deposit the solder only on a desired portion without selectively applying the solder paste, and to spare the circuit board. The throughput of solder processing is improved. Further, since it is possible to accurately cope with a substrate having a fine pad or wiring pitch, it can be applied to a pre-soldering process of a circuit substrate on which a highly integrated IC is mounted.

Further, the non-eutectic solder used in the present invention is softer than the eutectic solder, and the solder is plastically deformed to cope with the strain caused by the temperature change when the circuit board is used. The occurrence of stress is small, and undesirable situations such as breakage and changes in element characteristics are avoided.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a process of an example.

[Explanation of symbols]

 1 Board 2 Pad 3 Solder paste 3a Solder grain 4 Bump 4a Solder liquid mass

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H01L 21/321

Claims (3)

[Claims] 1. (a) A soft wax material powder having a non-eutectic composition and having a grain size of 1/5 or less of a minimum distance between pre-solder surfaces,
And (b) a solder paste comprising a flux vehicle containing a resin for enhancing the wettability of the metal surface to be brazed and the wax melt and an organic solvent dissolving the resin is prepared, and the solder paste is applied to the entire surface of the substrate. A preliminary soldering method, which comprises applying and reflowing in a steam bath at a temperature not higher than the boiling point of the organic solvent. 2. The solder paste used in the pre-soldering method according to claim 1, wherein the powder of the soft solder material is a powder of In-40% Pb alloy having a particle size of 20 μm or less, and the flux vehicle is 67 wt%. A solder paste comprising rosin, 32 wt% diethylene glycol monobutyl ether and 1 wt% hydrogenated castor oil. 3. The preliminary soldering method according to claim 1, wherein the solder paste according to claim 2 is used to perform the reflow treatment in the steam bath in a 215 ° C. Fluorinert saturated steam bath. Soldering method.